Fuel cells have been proposed as a power source of the future for automobiles and other industrial applications. One known fuel cell design is the PEM fuel cell that includes a “membrane-electrode assembly” comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, having very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles.
The membrane-electrode assembly is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, and may contain appropriate flow channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 & O2/air) over the surfaces of the respective anode and cathode.
A bipolar PEM fuel cell includes a plurality of the membrane-electrode-assemblies stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar or separator plate or septum. The separator or bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and each bipolar plate electrically conducts current between the adjacent cells. Contact elements at the ends of the stack are referred to as end, terminal, or collector plates. These terminal collectors contact a conductive element sandwiched between the terminal bipolar plate and the terminal collector plate.
There are competing interests in the design of these separator plates, and thus it is common for separator plates to be formed with different attributes reflective of the various design considerations. For example, it is desirable to maintain a low contact resistance at the interface of the separator plates to promote electrical conductivity, and thus greater efficiency of the fuel cell stack and more stable operation at low power conditions. For this reason it is important that at least a portion of the separator plate have electrically conductive characteristics. It is also important to produce a separator plate that is strong enough to withstand the pressure typically exerted upon the fuel cell stack during operation, as well as any handling during transit or assembly of the stack. Additionally it is important for the separator plate to resist corrosion, as the fuel cell stack environment generally can promote corrosion in certain materials. For these reasons it is known in the art to use either a composite plate having a polymeric base material which is relatively strong, ductile, and resistant to corrosion, blended with carbon particles or other electrically conductive elements which decrease the contact resistance of the separator plate or a metal substrate with a conductive, corrosion resistant coating.
For composite separator plates a lower polymeric content is desired to maximize the electrical conductivity of the plate. However, this generally results in a brittle separator plate prone to breaking during operation or handling of the fuel cell. A higher polymeric content may be used to maintain adequate plate ductility and resilience. Still, the compression molding process commonly used to form separator plates tends to cause the formation of a thin layer of polymeric resin on the outside of the formed plate. During the forming process the polymeric material tends to accumulate near the mold, around the outside surface of the separator plate. This thin, outer layer of polymeric resin material is low in electrical conductivity, and therefore tends to further increase the contact resistance of the separator plate.
This thin, outer layer of resin material of the polymeric separator plate may be removed by grinding, sanding, machining or some other mechanical means prior to installation of the separator plates in the fuel cell stack. There are still disadvantages to removing this outer layer of material through mechanical processes. These processes are relatively harsh operations for such delicate plates, and can easily result in breakage of the plates which are subjected to them. Even when performed properly, these operations tend to leave the plate with grooves or scratches and compromise the integrity of the conductive carbon particles that are left on the resulting surface. These carbon particles, which are important to maintaining the electrical conductivity of the plate, will tend to become loose and rub off of the plate as a result of grinding or sanding, leaving small voids on the surface of the plate. The scratches, grooves, and voids effectively reduce the electrical conductivity of the plate, and thus decrease the overall efficiency of the fuel cell stack. Alternately, the carbon particles may become depressed below the contact surface of the plate.
For metal separator plates, an oxide coating or layer can be formed on the surface to reduce the composite effects on the plate structure. However, this oxide layer tends to increase the contact resistance of the plate. Additionally, certain coatings which have hydrophilic properties can be used to improve stability at a wide range of operating conditions; however, these types of coatings usually increase the contact resistance of the plate.
Accordingly, there exists a need in the relevant art to provide a separator plate and method of manufacture that is capable of maintaining a high degree of strength while minimizing contact resistance. Additionally, there is a need in the relevant art to provide a separator plate and method of manufacture that overcomes the disadvantages of the prior art.